The use of wind turbines has become a common way of generating electricity and the size and performance of the turbines have increased. The wind turbines typically include multiple blades which transform wind energy into rotational motion of a drive train to thereby produce electricity. Existing turbines are controlled by computerised controllers which can amend various settings to bring the turbine to an optimum with regards to power production, load on the blades and drive train and general wear on the turbine.
Often, the blades can rotate about their longitudinal axis and thereby convert a varying degree of the wind energy. This activity is referred to as “pitching”, and in a traditional wind turbine, pitching is controlled such that the wind turbine utilises as much of the available wind energy until a rated power production is reached. If the wind turbine has reached the rated power production and the available wind energy increases further, the blades are pitched away from the optimal situation to maintain the rated power production. At a certain cut-out wind energy intensity, the blades are pitched to a position where transformation between wind energy and rotational motion is prevented. This is often referred to as “feathering”. The rotation stops and the control system waits for a decrease in wind intensity before reinitiating production by pitching back the blades from the feathered position.
The loads on the wind turbine structure are highly dependent on the climate conditions in which the turbine operates and the size of the major components e.g. blades. Different control algorithms are deployed today on the wind turbines to reduce the loads based on climate conditions.
Asymmetrical loadings across a turbine rotor are responsible for a significant contribution to fatigue loads. Asymmetrical loadings are caused e.g. by wind shear, resulting in tilt and yaw moments on the turbine rotor. Such asymmetrical loadings on the turbine rotor could be detected based on conditions, e.g. blade bending, experienced individually by each blade. Today wind turbine controllers are sometimes adapted to reduce or eliminate such tilt and yaw moments on the wind turbine rotor by controlling the pitch of each blade separately. Sometimes, this is referred to as Tilt and Yaw Control (TYC). In practise, the asymmetric loads are balanced by cyclic pitching of the blades based on estimated/calculated tilt and yaw moment on the rotor.
With the growing size of wind turbines, spatial wind distribution over the turbine rotor becomes more significant. As an example, there may exist a region of high wind speed at the upper region of the area defined by the rotor plane as shown in FIG. 1. This high wind speed region may be small enough to fit into the area between two blades.
In such a scenario, the rotor torque and thrust as well as tilt and yaw moments vary as a function of the position of the blade (i.e. rotor azimuth angle). While rotating, the upwards pointing blade enters the high wind speed area and leaves it, before the next blade does the same. This spatial wind speed distribution creates rotor loads which are pulsating or oscillating 3 times during one revolution. Such loads are also known as 3p loads and contribute to the overall asymmetrical loadings on the rotor. The 3p loads are also referred to as high order harmonics as they have frequency higher than the revolution of the rotor (1p).
It is desirable to provide a method and a system to eliminate or minimize such 3p loads on the rotor.